The present invention is directed to catalyst compositions and processes for reacting toluene and methanol to form styrene.
Styrene is currently commercially produced from benzene in a two-step process. In the first step benzene is alkylated with ethylene to form ethylbenzene, and in the second stop, the ethylbenzene is dehydrogenated to form styrene.
For example, the alkylation of aromatic compounds with olefins, alkyl halides and alcohols in the presence of a rare earth metal (including cerium) modified X- or Y-type zeolite is broadly disclosed in U.S. Pat. No. 3,251,897. Such alkylations are non-specific to styrene, the predominant reaction disclosed being benzene+ethylene to form ethylbenzene. Thus, such zeolite catalyzed reactions can be employed to make ethylbenzene in the first stage of conventional styrene synthesis.
One of the known alternative routes for forming styrene involves the oxidative coupling of toluene to form 1, 2-diphenyl ethylene (stilbene) followed by the disproportionation of the stilbene with ethylene in the presence of a catalyst to form styrene. The economic significance of the overall process scheme of the toluene-stilbene-styrene route resides in the fact that styrene can be produced from 0.5 mole of ethylene and one mole of toluene. This compares with the conventional ethylbenzene route wherein styrene is produced from one mole of ethylene and one mole of benzene.
In light of the rising costs of benzene and ethylene and the environmental problems of benzene, toluene-based processes will become a more attractive route than the existing benzene-based process for styrene manufacture.
Representative catalysts employed in the toluene to stilbene route for styrene synthesis are metal oxides such as those disclosed in U.S. Pat. Nos. 3,694,518; 3,739,038; 3,868,427; 3,965,206; 3,980,580; 4,091,044; 4,183,828; 4,243,825; 4,247,727; 4,254,293; 4,255,602; 4,255,603; 4,255,604; 4,268,703; 4,268,704; 4,278,824; 4,278,825; and 4,278,826 all assigned to Monsanto.
Commonly assigned U.S. patent application Ser. No. 405,803 now U.S. Pat. No. 4,429,174, filed Aug. 6, 1982 by H. Teng and I. Huang employs a faujasite zeolite modified with Li, K, Rb or cesium cations and at least one promoter selected from the group consisting of B, P, Pb, Cu, Zn, Ni, O, and Fe for the toluene to stilbene route.
A separate and distinct alternative route to styrene from toluene involves the alkylation of the side chain of toluene with methanol or formaldehyde by contact of these reactants with X- or Y-type zeolites, as described in Yashima et al in the Journal of Catalysis, Vol. 26, 303-312 (1972). More specifically, it is disclosed therein that alkylation of the methyl group of toluene to form styrene and ethylbenzene is effected by Na, K, Rb or Cs exchanged X- or Y-type zeolites, whereas Li exchanged zeolites of the same type effected predominantly the alkylation of the benzene ring of toluene to form xylenes. Yashima et al interpret their results as suggesting that xylene production is attributable to the acidity of the catalyst, whereas styrene and ethylbenzene formation is attributable to the basicity of the catalyst. At page 309 of Yashima et al, the authors discuss the effect on catalyst activity as a function of the percentage of ion exchange of a potassium exchanged X-type zeolite. The data presented at Table 3 therein indicates that styrene yield increases up to about a 60% potassium exchange but levels off at higher percentages of such potassium exchange. Yashima et al conclude that while the yield of C.sub.8 aromatics increases substantially with the percentage of K ion exchange up to about 60%, no marked increase is observed above this level of exchange. Yashima et al also conclude that a cesium exchanged X-type zeolite has a lower activity for toluene alkylation than, for example, a potassium exchanged X-type zeolite because of partial destruction of the zeolite crystallinity in the cesium exchanged zeolite. Yashima et al do not test or prepare a CS/K exchanged zeolite.
Sidorenko et al in the article "Condensation of Toluene and Methanol on Synthetic Zeolites Exchanged with Alkali Ions", Dokl. Akad. Nauk SSSR, Vol. 173 No. 1:132-34 (1967), have proposed a mechanism for the alkylation of toluene with methanol using alkali metal exchanged X- and Y-type zeolites wherein methanol is converted to formaldehyde which then reacts with toluene to produce styrene and ethylbenzene. Sidorenko et al test the following alkali metal exchanged type-X or type-Y zeolites: Li/Na-X, K/Na-X, Li/Na-Y, K/Na-Y, Rb/Na-Y, Cs/Na-Y, Rb/Na-X; but do not test K/Cs/Na-X or -Y type zeolites.
Since alkali metal exchanged zeolites are capable of catalyzing a variety of reactions and therefore produce a variety of by-products, the selectivity of the toluene to styrene is very low when conducting the process in accordance with Yashima et al or Sidorenko et al.
In an effort to improve the selectivity of the toluene/methanol alkylation reaction to styrene, Unland et al, U.S. Pat. No. 4,140,726 (a division of U.S. Pat. No. 4,115,424) describe the use of an X- or Y-type zeolite which has been modified by a cation exchange with one or more of potassium, rubidium and cesium and impregnated with boron or phosphorus. At Col. 3, lines 49 et seq. it is disclosed that (1) in theory only 81% of the sodium in type-X zeolite and 71% of the sodium in type-Y zeolite is exchangable with one or more of potassium, rubidium or cesium; (2) usual exchange procedures do not readily produce Na exchanges above about 60%; and (3) no improvement is observed above about a 60% Na exchange. Furthermore, Unland et al never actually prepare or test a K/Cs/Na-X or -Y type dual ion exchanged zeolite with or without B and/or phosphorus.
Itoh et al report in J. of Catalysis, Vol. 72, p. 170 (1981) the use of Rb, K, Li cation exchanged X-type zeolites, such as Rb/Li-X, Rb-X and Rb/K-X, for the side chain alkylation of p-xylene with methanol to produce p-methylstyrene and p-ethyltoluene. A maximum 68 mole % conversion of methanol with mole % yields of 5.3% (p-methylstyrene) and 2.7% (p-ethyltoluene) are disclosed. Itoh et al, however, do not prepare or test a K/Cs/Na-X or Y-type dual ion exchanged zeolite.
Japanese Patent Application Publication No. Sho 57-68144 published April 26, 1982 is directed to catalyst for styrene synthesis which comprises a zeolite of the faujasite class having at least 20% of the sodium cations present therein exchanged with cesium, potassium or rubidium and which has been treated to impregnate therein one or more divalent or trivalent metal salts of boric or phosphoric acid, the metal of said salt disclosed as being selected from magnesium, calcium, aluminum, magnanese, iron, cobalt, nickel, copper and zinc. In Comparative Example 4 thereof, a Cs exchanged X-type zeolite is impregnated with K.sub.3 PO.sub.4. This catalyst is used for comparative purposes and produces a methanol conversion of 71%, a styrene and ethylbenzene selectivity of 42.1% and a styrene to styrene+ethylbenzene ratio of 0.56. This Japanese patent publication refers to Unland et al, U.S. Pat. No. 4,115,424 for its description of the method of effecting ion exchange. Thus, the limitations on the percentage and effect of exceeding a 60-65% Na exchange of Unland et al are, in effect, incorporated into the Japanese patent publication. Futhermore, it is noted that the amount of potassium in the phosphate salt employed in the impregnation of the zeolite of Comparative Example 4 of the Japanese patent publication is determined by the amount of phosphorus sought to be impregnated into the zeolite not on the amount of sodium sought to be replaced in the zeolite by potassium.
U.S. Pat. No. 2,882,244 discloses the composition and preparation of zeolite X. At Col. 6, lines 15 et seq. it is disclosed that the adsorbents contemplated in this patent include not only the sodium form of the zeolite but also crystalline materials obtained from such zeolite by partial or complete replacement of sodium with other cations. At Col. 7, lines 28 et seq. it is stated that "by varying the concentration of the zinc or other exchange ion in solution, or by varying the time allowed for ion exchange, or by varying the temperature, the exchange ion may replace up to nearly 100% of the sodium ions".
Contrary to the above assertions, it is known that not all cations can effect a complete sodium exchange, cesium cations being one example (See, Unland et al). Thus, except by specific example, this patent does not teach which of the numerous potential cations disclosed therein can, in fact, effect complete or substantially complete ion exchange of sodium. Futhermore, in the examples, neither potassium nor cesium is employed in an actual exchange procedure resulting in complete or substantially complete sodium replacement. This patent does not suggest the use of any of the zeolites disclosed therein for the side chain alkylation of toluene and the like reactions.
U.S. Pat. No. 3,251,897 discloses the use of X- or Y-type zeolites for direct alkylation of aromatic compounds, e.g. benzene is reacted with ethylene to form ethylbenzene. As discussed above, this reaction is completely different from the side chain alkylation of alkylated aromatic compounds. The zeolites employed in the process of this patent are subjected to an exchange with rare earth cations. Such rare earth exchanges result in a sodium content of between about 5 and about 0.2 wt. based on the zeolite weight. The use of alkali metal exchanges is not disclosed.
Howard S. Sherry reported in his article "The Ion-Exchange Properties of Zeolite. I. Univalent Ion Exchange in Synthetic Faujasite", J. of Physical Chemistry, Vol. 70, pp. 1158-1168 (1966) the results of a study of the ion exchange of Linde X and Y zeolites. From ion-exchange isotherm data describing the exchange of Li, K, Rb, Cs, Ag, or Tl ions into Linde Na-X; he concluded that below a 40% replacement of Na ion the selectivity series is Ag&gt;&gt;Tl&gt;Cs.gtoreq.Rb&gt;K&gt;Na&gt;Li, e.g. potassium is preferred over sodium. However, above a 40% replacement of the Na ion the selectivity series becomes Ag&gt;&gt;Tl&gt;Na&gt;K&gt;Rb.gtoreq.Cs&gt;Li, e.g. sodium is preferred over potassium. Thus, it would appear difficult at high degrees of ion-exchange, to replace sodium with potassium ions. All of the exchanges conducted in Sherry are mono exchanges, i.e., only a single ion is exchanged for sodium in each exchange experiment. When conducting the exchanges on Linde Na-Y zeolites, a preference for Na over K is also described. Sherry also discusses the limitations of a cesium exchange to replace sodium.
Japanese Kokai 52-133, 932 published Nov. 9, 1977, discloses the use of a catalyst, formed by impregnating activated carbon with oxides of potassium, rubidium, cesium, or francium and mixtures thereof, for alkylating side chains of alkyl aromatic compounds with methanol.
Sodesawa et al, "A Study of Catalysis by Metal Phosphates V. the Alkylation of Toluene with Methanol over Metal Phosphate Catalysts", Bulletin of the Chemical Society of Japan, Vol. 52(8) pp. 2431-2432 (1979) disclose the use of catalysts, for the subject conversion reaction, of Ca.sub.3 (PO.sub.4).sub.2 or K.sub.3 PO.sub.4 supported on active carbon gave more ethylbenzene than the use of MgO.
Russian Patent No. 272299 discloses a process for alkylating toluene with formaldehyde using a sodium based type-X zeolite which has been partially exchanged with potassium, rubidium, or cesium.
From the above prior art discussion, it is observed that basic sites on the X- or Y-type zeolites are believed to be important for the side chain alkylation of toluene with methanol. Yashima et al recognized that the basicity of such zeolites depends of the basicity of the alkali metal cation in the zeolite, i.e. arranging the alkali metals in their increasing order of basicity they conclude Na&lt;Rb&lt;Cs. However, Yashima et al teach away from using cesium to control basicity of the zeolite, because of alleged destruction of the zeolite crystallinity by a cesium exchange. Even when one attempts to use cesium to control zeolite basicity, there are limits to the degree to which one can replace sodium with cesium cations as reported by Unland et al. This was not considered a problem by Unland et al since they failed to appreciate that any practical benefit could be obtained at exchange rates above about 60% of the sodium. Consequently, conventional wisdom in this area has been not even to attempt to exceed a sodium exchange rate above 60%.
Notwithstanding the above, the search has continued for catalyst compositions capable of improving the conversion and/or styrene selectivity of toluene side chain alkylation reactions with methanol. The present invention was developed in response to this search.